Surface oxidised nickel-iron metal anodes for aluminium production

ABSTRACT

An anode for the electrowinning of aluminium by the electrolysis of alumina in a molten fluoride electrolyte has an electrochemically active integral outside oxide layer obtainable by surface oxidation of a metal alloy which consists of 20 to 60 weight % nickel; 5 to 15 weight % copper; 1.5 to 5 weight % aluminium; 0 to 2 weight % in total of one or more rare earth metals, in particular yttrium; 0 to 2 weight % of further elements, in particular manganese, silicon and carbon; and the balance being iron. The metal alloy of the anode has a copper/nickel weight ratio in the range of 0.1 to 0.5, preferably 0.2 to 0.3.

FIELD OF THE INVENTION

This invention relates to surface oxidised nickel-iron metal anodes forthe electrowinning of aluminium by the electrolysis of alumina dissolvedin a molten fluoride-containing electrolyte, an aluminium electrowinningcell with such an anode and its use to produce aluminium.

BACKGROUND ART

Using non-carbon anodes in aluminium electrowinning cells shoulddrastically improve the aluminium production process by reducingpollution and the cost of aluminium production. Many attempts have beenmade to use oxide anodes, cermet anodes and metal-based anodes foraluminium production, however they were never adopted by the aluminiumindustry.

U.S. Pat. Nos. 6,248,227 and 6,436,274 (both de Nora/Duruz) disclose anon-carbon, metal-based slow-consumable anode of a cell for theelectrowinning of aluminium that self-forms during normal electrolysisan electrochemically-active oxide-based surface layer. The rate offormation of this layer is maintained substantially equal to its rate ofdissolution at the surface layer/electrolyte interface therebymaintaining its thickness substantially constant.

A different approach was taken in WO 00/06802 (Duruz/de Nora/Crottaz)where anodes comprising a transition metal-based oxide active surface ofiron oxide, cobalt oxide, nickel oxide or combinations thereof, werekept dimensionally stable during electrolysis by continuously orintermittently feeding to the electrolyte a sufficient amount of aluminaand transition metal species that are present as oxides at the anodesurface.

WO 00/40783 (de Nora/Duruz) further describes the use of HSLA steel witha coherent and adherent oxide surface as an anode for aluminiumelectrowinning.

Nickel-iron alloy anodes with various additives are further described inWO 00/06803 (Duruz/de Nora/Crottaz), WO 00/006804 (Crottaz/Duruz), WO01/42534 (de Nora/Duruz), WO 01/42535, (Duruz/de Nora), WO 01/42536(Duruz/Nguyen/de Nora) and WO02/083991 (Nguyen/de Nora).

SUMMARY OF THE INVENTION

An object of the invention is to provide a nickel-iron alloy-based anodefor aluminium electrowinning having a long life, which anode during usedoes not contaminate the product aluminium beyond an acceptable level.

The invention relates to an alloy-based anode for the electrowinning ofaluminium by the electrolysis of alumina in a molten fluorideelectrolyte. The anode has an electrochemically active integral outsideoxide layer obtainable by surface oxidation of a metal alloy having acomposition adjusted to achieve the effect described below. This metalalloy consists of:

-   -   20 to 60, preferably 35 to 60, weight % nickel;    -   5 to 15, preferably 6 to 12, weight % copper;    -   1.5 to 5, preferably 1.5 to 4, weight % aluminium;    -   0 to 2, preferably 0.2 to 0.5, weight % in total of one or more        rare earth metals, in particular yttrium;    -   0 to 2, usually 0.5 to 1.5, weight % of further elements, in        particular manganese, silicon and carbon; and    -   the balance being iron,        the metal alloy having a copper/nickel weight ratio in the range        of 0.1 to 0.5, preferably 0.2 to 0.3.

When such a metal alloy is exposed to an oxidising atmosphere atelevated temperature, e.g. above 600° C., typically 700° to 1000° C.,for a duration of up to 36 hours depending on the temperature, and/orduring use in an aluminium production cell, the iron migrates from anouter part to the surface where it is oxidised.

When the anode's alloy is oxidised before use, the integral oxide layerformed thereon usually consists essentially of iron oxides and up 30weight % nickel oxide, in particular from 1 to 10, weight %.

Whether or not the alloy is oxidised before use, the integral oxidelayer typically comprises during use in a cell an iron-rich outerportion which consists essentially of non-stoichiometric well conductiveiron oxide (FeO_(x)) and nickel oxide in a metal equivalent weight ratiothat is at least 9 iron for 1 nickel, and an iron-rich inner portionwhich consists essentially of a mixture of oxides of iron, nickel,copper and aluminium which are present in metal equivalent weightpercentages of 65 or 70 to 80% iron, 15 to 25 or 30% nickel 2 to 3%copper and up to 1% aluminium. Usually, the outer portion of theintegral oxide layer makes about ⅓ of the thickness of the layer,whereas the inner portion makes about ⅔ of the thickness of the integraloxide layer.

Underneath the electrochemically active oxide surface, the(iron-depleted) alloy outer part is rich in copper and nickel metal in aratio derived from the nickel-copper ratio of the alloy's nominalcomposition and contains a limited amount of iron metal. Thecopper-nickel outer part controls the iron diffusion from inside theanode to its electrochemically active surface so as to compensate slowdissolution of iron oxides from the anode's active surface into theelectrolyte while it prevents excessive iron diffusion to the anode'ssurface and dissolution into the electrolyte of an excess of iron oxidefrom the anode's surface, which would lead to premature iron depletionof the anode's alloy and unnecessary and unwanted contamination of theproduct aluminium.

Typically, the nickel-copper metal outer part has a nickel/copper weightratio in the range of 1.8 to 4 upon heat treatment and/or during use ina cell.

The small amount of aluminium contained in the anode's alloy diffuses tothe grain joints of the nickel-iron alloy inside the anode where it isoxidised to form a partial barrier against oxygen diffusion into thealloy's grains and iron diffusion therefrom. Thus, the combined effectof the alloy's aluminium on the one hand and of the anode'snickel-copper outer part on the other hand leads to a control of thesupply of iron to the anode's active surface.

Small amounts of rare earth metals, such as yttrium, are preferably usedin the anode's alloy to improve the anchorage of the integral oxidelayer on the nickel-copper outer part. For example, the metal alloycontains 0.3 to 0.4 weight % yttrium.

The anode's metal alloy can contain 16 to 73.5 weight % iron, usuallyfrom 20 to 70 weight %. In particular in this case, the nickel/ironweight ratio can be in the range of 0.3 to 2.5.

In one embodiment the anode's metal alloy contains 30 to 70 weight %iron, preferably 40 to 60 weight %. Especially in this case, thenickel/iron weight ratio can be in the range of 0.3 or 0.4 to 1.5,preferably 0.7 to 1.2.

In another embodiment, the anode's metal alloy contains 20 to 40 weight% iron, preferably 25 to 35 weight %. Particularly in this case, thenickel/iron weight ratio may be in the range of 1.5 to 3, preferably 2to 2.5.

Especially when the anode is used with an electrolyte in a reducedtemperature range, e.g. from 850-880° to 940° C., the anode's alloypreferably contains at least one of the metals nickel, copper, aluminiumand iron in the respective amounts: 35 to 50 weight % nickel; 6 to 10weight % copper; 3 to 4 weight % aluminium; and 32 to 56 weight % iron,in particular 35 to 55 weight % iron. For instance, the alloy contains:35 to 50 weight % nickel; 6 to 10 weight % copper; 3 to 4 weight %aluminium; 32 to 56 weight % iron, in particular 35 to 55 weight % iron;and 0 to 4 weight % in total of further elements, i.e. the rare earthmetals plus the abovementioned further elements.

Especially when the anode is used with an electrolyte in a highertemperature range, e.g. from 910° to 960° C. such as 930° to 950° C.,the anode's alloy preferably contains at least one of the metals nickel,copper, aluminium and iron in the respective amounts: 50 to 60 weight %nickel, in particular 55 to 60 weight %; 7 to 12 weight % copper; 1.5 to3 weight % aluminium; and 21 to 41.5 weight % iron, preferably 21 to36.5 weight %. In particular, the alloy contains: 50 to 60 weight %nickel, in particular 55 to 60 weight %; 7 to 12 weight % copper; 1.5 to3 weight % aluminium; and 21 to 41.5 weight % iron, preferably 21 to36.5 weight %; and 0 to 4 weight % in total of further elements (therare earth metals plus the abovementioned further elements).

Advantageously, the metal alloy contains manganese to trap andsolubilise in the alloy sulphur that can be present as an impurity inthe electrolyte. In the absence of manganese, sulphur combines withnickel to from NiS instead of MnS and migrates to the grain joints ofthe alloy and impairs its properties. The alloy preferably containsmanganese in an amount of less than 1 weight %, in particular from 0.2to 0.5 weight %.

When the metal alloy is cast, especially to produce complex shapes,silicon can be used to lower the viscosity of the alloy and enhance itscastability. It is not unusual to find 0.2 to 0.7 weight % silicon inthe metal alloy when it is cast.

Furthermore, to avoid oxidation of the metal alloy when it is cast,carbon can be used to trap any oxygen to which the alloy may be exposedduring casting. Therefore, residual amounts of carbon, typically 0.01 to0.2 weight %, is commonly found in such alloys.

For example, the metal alloy consists of 41 to 49 weight % nickel, 41 to49 weight % iron, 6 to 8 weight % copper, 2.5 to 3.5 weight % aluminiumand 0 to 2 weight % in total of further elements (the rare earth metalsplus the abovementioned further elements). The metal alloy can alsoconsist of 33 to 39 weight % nickel, 49 to 59 weight % iron, 6 to 8weight % copper, 2.5 to 3.5 weight % aluminium and 0 to 2 weight % intotal of further elements (the rare earth metals plus the abovementionedfurther elements).

The anode's metal alloy can contain 0 to 1.5 weight % in total offurther elements (the rare earth metals plus the abovementioned furtherelements), preferably no more than about 1 weight %.

In another embodiment, the anode's alloy consists of 56 to 58 weight %nickel, 28 to 32 weight % iron, 9 to 11 weight % copper, 1.5 to 2.5weight % aluminium and 0 to 1 or 1.5 weight % in total of furtherelements (the rare earth metals plus the abovementioned furtherelements).

The anode is preferably covered with a protective coating on theintegral oxide layer, in particular a protective oxide coating. Suitableoxide coatings may contain iron oxide such as hematite (Fe₂O₃), inparticular a coating made of hematite and at least one oxide selectedfrom oxides of titanium, yttrium, ytterbium and tantalum as disclosed inPCT/IB02/02973 (Nguyen/de Nora). Other suitable coatings can be used toprotect the anode's alloy, in particular oxide coatings as disclosed inWO99/36594 (de Nora/Duruz), U.S. Pat. No. 6,077,415 (Duruz/de Nora),U.S. Pat. No. 6,103,090 (de Nora) U.S. Pat. No. 6,361,681 (deNora/Duruz), U.S. Pat. No. 6,365,018 (de Nora), or cerium-basedcoatings, especially for use in an electrolyte in a higher temperaturerange, e.g. in the range of 910° to 960° C., for example thecerium-based coatings disclosed in U.S. Pat. No. 4,614,569(Duruz/Derivaz/Debely/Adorian), U.S. Pat. No. 4,966,674(Bannochie/Sheriff), U.S. Pat. Nos. 4,683,037 and 4,680,094 (both in thename of Duruz), U.S. Pat. Nos. 4,960,494, 4,956,068 and 5,069,771 (allin the name of Nyguen/Lazouni/Doan), and WO02/070786 (Nguyen/de Nora)and WO02/083990 (de Nora/Nguyen).

Unless specified otherwise, all the above mentioned metal percentages ofthe alloy refer to the nominal alloy composition, i.e. before any heattreatment or use in a cell.

The invention relates also to an aluminium electrowinning cellcomprising at least one anode as described above.

Advantageously, the cell comprises an aluminium-wettable cathode, inparticular a drained cathode. Suitable aluminium-wettable cathodematerials are disclosed in WO01/42168 (de Nora/Duruz), WO01/42531(Nguyen/Duruz/de Nora), WO02/070783 (de Nora), WO02/096830(Duruz/Nguyen/de Nora) and WO02/096831 (Nguyen/de Nora). Suitabledrained cathode designs are disclosed in U.S Pat. No. 5,683,559 (deNora) and U.S. Pat. No. 6,258,246 (Duruz/de Nora), and in PCTapplications WO99/02764, WO99/41429 (both de Nora/Duruz), WO00/63463 (deNora), WO01/31086 (de Nora/Duruz), WO01/31088 (de Nora), WO02/070785 (deNora), WO02/097168 (de Nora) and WO02/097169 (de Nora).

Another aspect of the invention relates to a method of electrowinningaluminium. The method comprises passing an electrolysis current in amolten electrolyte containing dissolved alumina between a cathode and ananode as described above to produce aluminium cathodically and oxygenanodically.

During cell operation, oxides of the anode's oxide layer may slowlydissolve in the electrolyte, the oxide layer being maintained by slowoxidation of the anode's metal alloy at the oxide layer/metal alloyinterface. Advantageously, the dissolution rate of the anode's oxides issubstantially equal to the oxidation rate of the metal alloy at theoxide layer/metal alloy interface, as taught in U.S. Pat. No. 6,248,227and WO00/06805 (both de Nora/Duruz).

Alternatively, dissolution of oxides of the anode's oxide layer can beinhibited, in particular prevented, by maintaining in the electrolyte anamount of alumina and iron species, preferably at a level close to or atsaturation, as disclosed in WO00/06802 (Duruz/de Nora/Crottaz).

Preferably, the electrolyte has a temperature which is maintainedsufficiently low to limit the solubility of iron species in theelectrolyte and the contamination of the product aluminium to anacceptable level. The electrolyte temperature of the cell may be in areduced temperature range, typically from 850° C. to 940° C., preferablybetween 880° C. and 930° C. Alternatively, the electrolyte temperaturemay be in a higher temperature range, typically in the range of 910° C.to 960° C., in particular from 930° C. to 950° C.

The electrolyte can contain sodium fluoride (NaF) and aluminium fluoride(AlF₃) in a molar ratio in the range from 1.2 to 2.4, in particular from1.4 to 1.9 with an electrolyte in a reduced temperature range and from1.7 to 2.3 with an electrolyte in a higher temperature range. Suitableelectrolyte compositions are disclosed in WO02/097168 (de Nora).

Advantageously, the electrolyte is continuously circulated from analumina feeding area where it is enriched with alumina to the anodewhere the alumina is electrolysed and from the anode back to the aluminafeeding area so as to maintain a high alumina concentration near theanode. Means for providing such a circulation are disclosed inWO99/41429 (de Nora/Duruz), WO00/40781, WO00/40781 and WO03/006716 (allde Nora).

A further aspect of the invention relates to an alloy, in particular foruse to produce an anode for the electrowinning of aluminium. The alloyconsists of:

-   -   20 to 60, preferably 35 to 60, weight % nickel;    -   5 to 15, preferably 6 to 12, weight % copper;    -   1.5 to 5, preferably 1.5 to 4, weight % aluminium;    -   0 to 2, preferably 0.2 to 0.5, weight % in total of one or more        rare earth metals, in particular yttrium;    -   0 to 2, usually 0.5 to 1.5, weight % of further elements, in        particular manganese, silicon and carbon; and    -   the balance being iron,        the alloy having a copper/nickel weight ratio in the range of        0.1 to 0.5, preferably 0.2 to 0.3.

The alloy can contain at least one of the metals nickel, copper,aluminium and iron in the respective amounts: 35 to 50 weight % nickel;6 to 10 weight % copper; 3 to 4 weight % aluminium; and 32 to 56 weight% iron, in particular 35 to 55 weight % iron. In particular, the alloycontains: 35 to 50 weight % nickel; 6 to 10 weight % copper; 3 to 4weight % aluminium; 32 to 56 weight % iron, in particular 35 to 55weight % iron; and 0 to 4 weight % in total of further elements (therare earth metals plus the abovementioned further elements).

The alloy may also contain at least one of the metals nickel, copper,aluminium and iron in the respective amounts: 50 to 60 weight % nickel,in particular 55 to 60 weight %; 7 to 12 weight % copper; 1.5 to 3weight % aluminium; and 21 to 41.5 weight % iron, preferably 21 to 36.5weight %. In particular, the alloy contains: 50 to 60 weight % nickel,in particular 55 to 60 weight %; 7 to 12 weight % copper; and 1.5 to 3weight % aluminium; 21 to 41.5 weight % iron, preferably 21 to 36.5weight %; and 0 to 4 weight % in total of further elements (the rareearth metals plus the abovementioned further elements).

Another aspect of the invention relates to an anode starter for theelectrowinning of aluminium having an outer part made of the alloydescribed above which is oxidisable before and/or during use to form anintegral electrochemically active oxide outer layer.

A further aspect of the invention relates to a component of an aluminiumelectrowinning cell, in particular an anode support member or a currentdistribution member. This cell component has an outer part made of thealloy described above which is oxidisable before and/or during use toform an integral oxide outer layer.

DETAILED DESCRIPTION

Examples of anode alloy compositions according to the invention aregiven in Table I, which shows the weight percentages of the indicatedmetals for each specimen A-R.

TABLE I Ni Fe Cu Al Y Mn Si C A 48 38 10 3 — 0.5 0.45 0.05 B 49 40 7 3 —0.5 0.45 0.05 C 36 50 10 3 — 0.5 0.45 0.05 D 36 50 10 3 0.35 0.3 0.30.05 E 36 53 7 3 — 0.5 0.45 0.05 F 36 53 7 3 0.35 0.3 0.3 0.05 G 48 3810 3 0.35 0.3 0.3 0.05 H 48 38 10 3 0.2 0.3 0.45 0.05 I 22 68 5.5 4 —0.25 0.2 0.05 J 22 69 5.5 3 — 0.25 0.2 0.05 K 42 42 12 2 1 0.5 0.45 0.05L 42 40 12.5 4 0.4 0.45 0.6 0.05 M 45 44 7 3 — 0.5 0.45 0.05 N 55 30 122 0.2 0.3 0.45 0.05 O 53 36 8 2.3 0.1 0.2 0.35 0.05 P 55 32 10 2 0.2 0.30.45 0.05 Q 57 30 10 2 0.2 0.3 0.45 0.05 R 59 27 10 3 0.2 0.3 0.45 0.05

The invention will be further described in the following Examples.

EXAMPLE 1

An anode rod of diameter 20 mm and total length 200 mm was prepared bycasting the composition of Sample A of Table I, using a sand mould. Theanode was oxidised in air for 24 hours at 700° C.

Electrolysis was carried out in a laboratory scale cell equipped withthis oxidised anode immersed to a depth of 50 mm in afluoride-containing molten electrolyte at 920° to 930° C. Theelectrolyte consisted of 16 weight % aluminium fluoride (AlF₃) and 7weight % alumina Al₂O₃ and 4 weight % CaF₂, the balance being cryolite(3NaF—AlF₃).

The current density was about 0.8 A/cm² at a cell voltage of 3.5 to 3.8V. The concentration of dissolved alumina in the electrolyte wasmaintained during the entire electrolysis by periodically feeding freshalumina into the cell.

After 150 hours electrolysis was interrupted and the anode extracted.Upon cooling the anode was examined externally and in cross-section.

The anode's outer dimensions had remained substantially unchanged.

The anode was covered with an external oxide scale having a thickness ofabout 50-100 micron. The oxide scale had an outer portion that consistedessentially of non-stoichiometric iron oxide (FeO_(x)) with smallamounts of nickel oxide (metal equivalent of about 90 weight % Fe and 10weight % Ni) at its surface which is electrochemically active duringuse. Below the outer portion, the external oxide scale had an innerportion that consisted essentially of a mixture of hematite (Fe₂O₃) andmixed oxides of nickel, iron and aluminium.

Underneath the oxide scale, the anode's alloy had become vermicular overa depth of about 1500 micron and contained 75 weight % nickel and 15weight % copper, the balance being essentially iron (below 10 weight %).The vermicular outer part of the alloy had elongated pores having adiameter of 3 to 5 micron and a length of 10 to 30 micron and containingoxides essentially of iron. Below the anode's vermicular part the alloywas non vermicular but had the same metal alloy composition as thevermicular outer part over a depth of about 50 micron followed by anunchanged inner part having the nominal composition of the alloy beforeheat treatment.

The alloy grain joints were oxidised all over the vermicular outer partand to a depth of about 100 micron therebelow.

EXAMPLE 1a

An anode rod of diameter 20 mm and total length 20 mm was prepared bycasting the composition of Sample B of Table I, using a sand mould. Theanode was oxidised in air for 24 hours at 700° C. and then tested in alaboratory scale cell as in Example 1.

Similar results were obtained as in Example 1 except that the wear rateof the anode had increased to about 1 mm per 100 hours of use.

EXAMPLE 2

An anode rod of diameter 20 mm and total length 200 mm was prepared bycasting the composition of Sample N of Table I, using a sand mould. Theanode was oxidised in air for 24 hours at 750° C.

Electrolysis was carried out in a laboratory scale cell equipped withthis oxidised anode immersed to a depth of 50 mm in afluoride-containing molten electrolyte at about 940° C. The electrolyteconsisted of 15 weight % aluminium fluoride (AlF₃) and 7 weight %alumina Al₂O₃ and 4 weight % CaF₂, the balance being cryolite(3NaF—AlF₃).

The current density was about 0.8 A/cm² at a cell voltage of 3.5 to 3.8V. The concentration of dissolved alumina in the electrolyte wasmaintained during the entire electrolysis by periodically feeding freshalumina into the cell.

After 200 hours electrolysis was interrupted and the anode extracted.Upon cooling the anode was examined externally and in cross-section.

The anode's outer dimensions had remained substantially unchanged.

The anode was covered with an external oxide scale having a thickness ofabout 50-100 micron. The oxide scale had an outer portion that consistedessentially of non-stoichiometric iron oxide (FeO_(x)) with smallamounts of nickel oxide (metal equivalent of about 70 weight % Fe and 30weight % Ni) at its surface which is electrochemically active duringuse. Below the outer portion, the external oxide scale had an innerportion that consisted essentially of a mixture of hematite (Fe₂O₃) andmixed oxides of nickel, iron and aluminium.

Underneath the oxide scale and over a depth of about 150 micron, theanode's alloy was nearly non-porous and contained about 70-75 weight %nickel and 20 weight % copper, the balance being essentially iron (below10 weight %). Therebelow, the anode's alloy had remained unchanged(nominal composition of sample N before heat treatment).

The alloy grain joints were nearly not oxidised, unlike those of Example1a.

EXAMPLE 3

An anode rod of diameter 20 mm and total length 200 mm was prepared bycasting the composition of Sample N of Table I, using a sand mould.

A slurry for the application of a protective coating onto the anode rodwas prepared by suspending a particle mixture of Fe₂O₃ particles (−325mesh, i.e. smaller than 44 micron) and TiO₂ particles (−325 mesh) incolloidal alumina (NYACOL® Al-20, a milky liquid with a colloidalparticle size of about 40 to 60 nanometer and containing 20 weight %colloidal particle and 80 weight % liquid solution) in a weight ratioFe₂O₃:TiO₂:colloid of 40:20:40. The pH of the slurry was adjusted at 4by adding a few drops of HNO₃ to avoid gelling of the slurry.

The anode rod was covered with several layers of this slurry using abrush. The applied layers were dried for 10 hours at 140° C. The driedlayers formed a coating of about 350-450 micron thick on the anode rod.

The anode rod was pre-heated over a molten electrolyte for an hour.During pre-heating at about 900°-950° C., the coating was furtherconsolidated by reactive sintering of the iron oxide and the titaniumoxide. During the pre-heating or at the latest at the beginning of usein the electrolyte, the coating became substantially continuous andthoroughly reacted forming a protective multiple oxide matrix of Fe₂O₃and TiO₂. Underneath the protective coating, an integral oxide scalemainly of iron oxide was grown from the alloy rod during the heattreatment and reacted with TiO₂ from the coating to firmly anchor thecoating to the anode rod. The reacted integral oxide scale containedtitanium oxide in an amount of about 10 metal weight %. Minor amounts ofcopper, aluminium and nickel were also found in the oxide scale (lessthat 5 metal weight % in total).

Electrolysis was carried out as in Example 2. The current density wasabout 0.8 A/cm² at a reduced cell voltage of 3.1 to 3.3 V.

After 200 hours electrolysis was interrupted and the anode extracted.Upon cooling the anode was examined and no significant change wasobserved.

Samples of the used electrolyte and the product aluminium were analysed.It was found that the electrolyte was nickel-free and the producedaluminium contained less than 300 ppm nickel. This demonstrated that theFe₂O₃—TiO₂ coating constituted an efficient barrier against nickeldissolution from the anode's alloy.

EXAMPLE 4

Anode rods can be prepared, as in Examples 1, 1a and 2, respectively, bycasting using sand moulds and oxidising in air the composition of TableI's Samples C to M and O to R, respectively, and as in Example 3 bycasting and coating the composition of Table I's Samples A to M and O toR. Thereafter, the anode rods can be tested in laboratory scale cells asin Examples 1 to 3.

EXAMPLE 5

Examples 1, 1a and 2 and their variations disclosed in Example 4 can berepeated without oxidation of the anode rods before use.

1. An alloy-based anode for the electrowinning of aluminium by the electrolysis of alumina in a molten fluoride electrolyte, having an electrochemically active integral outside oxide layer obtainable by surface oxidation of a metal alloy which consists of: 20 to 60, preferably 35 to 60, weight % nickel; 5 to 15, preferably 6 to 12, weight % copper; 1.5 to 5, preferably 1.5 to 4, weight % aluminium; 0 to 2, preferably 0.2 to 0.5, weight % in total of one or more rare earth metals, in particular yttrium; 0 to 2, usually 0.5 to 1.5, weight % of further elements, in particular manganese, silicon and carbon; and the balance being iron, in an amount 25 to 70, preferably 40 to 60, weight %, and which has a copper/nickel weight ratio in the range of 0.1 to 0.5, preferably 0.2 to 0.3.
 2. The anode of claim 1, wherein said metal alloy has a nickel/iron weight ratio in the range of 0.3 to 1.5, preferably 0.7 to 1.2.
 3. The anode of claim 1, wherein said metal alloy has a nickel/iron weight ratio in the range of 1.5 to 2.4.
 4. The anode of claim 1, wherein said metal alloy contains at least one of the metals nickel, copper, aluminium and iron in the respective amounts: 35 to 50 weight % nickel; 6 to 10 weight % copper; 3 to 4 weight % aluminium; and 32 to 56 weight % iron, in particular 35 to 55 weight % iron.
 5. The anode of claim 4, wherein said metal alloy contains: 35 to 50 weight % nickel; 6 to 10 weight % copper; 3 to 4 weight % aluminium; 32 to 56 weight % iron, in particular 35 to 55 weight % iron; and 0 to 4 weight % in total of further elements.
 6. The anode of claim 1, wherein said metal alloy contains at least one of the metals nickel, copper, aluminium and iron in the respective amounts: 50 to 60 weight % nickel, in particular 55 to 60 weight %; 7 to 12 weight % copper; 1.5 to 3 weight % aluminium; and 25 to 41.5 weight % iron, in particular 25 to 36.5 weight %.
 7. The anode of claim 6, wherein said metal alloy contains: 50 to 60 weight % nickel, in particular 55 to 60 weight %; 7 to 12 weight % copper; 1.5 to 3 weight % aluminium; 25 to 41.5 weight % iron, in particular 25 to 36.5 weight %; and 0 to 4 weight % in total of further elements.
 8. The anode of claim 1, wherein said metal alloy contains yttrium in an amount of 0.3 to 0.4 weight %.
 9. The anode of claim 1, wherein said metal alloy contains manganese in an amount of less than 1 weight %, in particular from 0.2 to 0.6 weight %.
 10. The anode of claim 1, wherein said metal alloy contains silicon in an amount of 0.2 to 0.7 weight %.
 11. The anode of claim 1, wherein said metal alloy contains carbon in an amount of 0.01 to 0.2 weight %.
 12. The anode of claim 1, wherein said metal alloy consists of 41 to 49 weight % nickel, 41 to 49 weight % iron, 6 to 8 weight % copper, 2.5 to 3.5 weight % aluminium and 0 to 2 weight % in total of further elements.
 13. The anode of claim 1, wherein said metal alloy consists of 33 to 39 weight % nickel, 49 to 59 weight % iron, 6 to 8 weight % copper, 2.5 to 3.5 weight % aluminium and 0 to 2 weight % in total of further elements.
 14. The anode of claim 1, wherein said metal alloy contains 0 to 1.5 weight %, preferably no more than about 1 weight %, in total of further elements.
 15. The anode of claim 1, wherein said metal alloy consists of 56 to 58 weight % nickel, 28 to 32 weight % iron, 9 to 11 weight % copper, 1.5 to 2.5 weight % aluminium and 0 to 1.5 weight % in total of further elements, preferably no more than 1 weight %.
 16. The anode of claim 1, comprising a protective coating on the integral oxide layer, in particular a protective oxide coating.
 17. An aluminium electrowinning cell comprising at least one anode as defined in claim
 1. 18. The cell of claim 17, comprising an aluminium-wettable cathode, in particular a drained cathode.
 19. A method of electrowinning aluminium comprising passing an electrolysis current in a molten electrolyte containing dissolved alumina between a cathode and an anode according to claim 1 to produce aluminium cathodically and oxygen anodically.
 20. The method of claim 19, wherein oxides of the anode's oxide layer slowly dissolve in the electrolyte, the oxide layer being maintained by slow oxidation of the anode's metal alloy at the oxide layer/metal alloy interface.
 21. The method of claim 20, wherein the dissolution rate of the anode's oxides is substantially equal to the oxidation rate of the metal alloy at the oxide layer/metal alloy interface.
 22. The method of claim 19, wherein dissolution of oxides of the anode's oxide layer is inhibited by maintaining in the electrolyte an amount of alumina and iron species, preferably at a level close to or at saturation.
 23. The method of claim 19, wherein the electrolyte has a temperature which is maintained sufficiently low to limit the solubility of iron species in the electrolyte and the contamination of the product aluminium to an acceptable level.
 24. The method of claim 23, wherein the electrolyte temperature is below 940° C., preferably from 880° C. to 930° C.
 25. The method of claim 23, wherein the cell comprises an anode whose said metal alloy contains at least one of the metals nickel, copper, aluminium and iron in the respective amounts: 35 to 50 weight % nickel; 6 to 10 weight % copper; 3 to 4 weight % aluminium; and 32 to 56 weight % iron, in particular 35 to 55 weight % iron.
 26. The method of claim 23, wherein the electrolyte temperature is from 910° C. to 960° C., preferably from 930° C. to 950° C.
 27. The method of claim 26, wherein the cell comprises an anode whose said metal alloy contains at least one of the metals nickel, copper, aluminium and iron in the respective amounts: 50 to 60 weight % nickel, in particular 55 to 60 weight %; 7 to 12 weight % copper; 1.5 to 3 weight % aluminium; and 25 to 41.5 weight % iron, in particular 25 to 36.5 weight %.
 28. The method of claim 26, wherein the cell comprises an anode whose said metal alloy contains: 50 to 60 weight % nickel, in particular 55 to 60 weight %; 7 to 12 weight % copper; 1.5 to 3 weight % aluminium; 25 to 41.5 weight % iron, in particular 25 to 36.5 weight %; and 0 to 4 weight % in total of further elements.
 29. The method of claim 26, wherein the cell comprises an anode whose said metal alloy consists of 56 to 58 weight % nickel, 28 to 32 weight % iron, 9 to 11 weight % copper, 1.5 to 2.5 weight % aluminium and 0 to 1.5 weight % in total of further elements, preferably no more than 1 weight %.
 30. The method of claim 23, wherein the cell comprises an anode whose said metal alloy contains: 35 to 50 weight % nickel; 6 to 10 weight % copper; 3 to 4 weight % aluminium; 32 to 56 weight % iron, in particular 35 to 55 weight % iron; and 0 to 4 weight % in total of further elements.
 31. The method of claim 23, wherein the cell comprises an anode whose said metal alloy consists of 41 to 49 weight % nickel, 41 to 49 weight % iron, 6 to 8 weight % copper, 2.5 to 3.5 weight % aluminium and 0 to 2 weight % in total of further elements.
 32. The method of claim 23, wherein the cell comprises an anode whose said metal alloy consists of 33 to 39 weight % nickel, 49 to 59 weight % iron, 6 to 8 weight % copper, 2.5 to 3.5 weight % aluminium and 0 to 2 weight % in total of further elements.
 33. The method of claim 19, wherein the electrolyte contains NaF and AlF₃ in a molar ratio in the range from 1.2 to 2.4.
 34. The method of claim 19, comprising continuously circulating the electrolyte from an alumina feeding area where it is enriched with alumina to the anode where the alumina is electrolysed and from the anode back to the alumina feeding area so as to maintain a high alumina concentration near the anode.
 35. An alloy, in particular for use to produce an anode for the electrowinning of aluminium, consisting of: 20 to 60, preferably 35 to 60, weight % nickel; 5 to 15, preferably 6 to 12, weight % copper; 1.5 to 5, preferably 1.5 to 4, weight % aluminium; 0 to 2, preferably 0.2 to 0.5, weight % in total of one or more rare earth metals, in particular yttrium; 0 to 2, usually 0.5 to 1.5, weight % of further elements, in particular manganese, silicon and carbon; and the balance being iron, in an amount 25 to 70, preferably 40 to 60, weight %, and which has a copper/nickel weight ratio in the range of 0.1 to 0.5, preferably 0.2 to 0.3.
 36. The alloy of claim 35, which contains at least one of the metals nickel, copper, aluminium and iron in the respective amounts: 35 to 50 weight % nickel; 6 to 10 weight % copper; 3 to 4 weight % aluminium; and 32 to 56 weight % iron, in particular 35 to 55 weight % iron.
 37. The alloy of claim 36, which contains: 35 to 50 weight % nickel; 6 to 10 weight % copper; 3 to 4 weight % aluminium; 32 to 56 weight % iron, in particular 35 to 55 weight % iron; and 0 to 4 weight % in total of further elements.
 38. The alloy of claim 35, which contains at least one metal from the group consisting of nickel, copper, aluminium and iron in the following amounts: 50 to 60 weight % nickel, in particular 55 to 60 weight %; 7 to 12 weight % copper; 1.5 to 3 weight % aluminium; and 25 to 41.5 weight % iron, in particular 21 to 36.5 weight %.
 39. The alloy of claim 38, which contains: 50 to 60 weight % nickel, in particular 55 to 60 weight %; 7 to 12 weight % copper; 1.5 to 3 weight % aluminium; 25 to 41.5 weight % iron, in particular 25 to 36.5 weight %; and 0 to 4 weight % in total of further elements.
 40. An anode starter for the electrowinning of aluminium having an outer part made of the alloy of claim 35 which is oxidisable before and/or during use to form an integral electrochemically active oxide outer layer.
 41. A component of an aluminium electrowinning cell, in particular an anode support member or a current distribution member, having an outer part made of the alloy of claim 35 which is oxidisable before and/or during use to form an integral oxide outer layer. 